This application claims benefits of Japanese Application No. 2001-351623 filed in Japan on Nov. 16, 2001, the contents of which are incorporated by this reference.
The present invention relates generally to a zoom lens and an electronic imaging system that incorporates the same, and more particularly to a zoom lens, the depth dimension of which is diminished by providing some contrivances to an optical system portion such as a zoom lens and an electronic imaging system using the same, such as a video or digital camera. According to the present invention, the zoom lens is also designed to be capable of rear focusing.
In recent years, digital cameras (electronic cameras) have received attention as cameras of the next generation, an alternative to silver-halide 35 mm-film (usually called Leica format) cameras. Currently available digital cameras are broken down into some categories in a wide range from the high-end type for commercial use to the portable low-end type. In view of the category of the portable low-end type in particular, the primary object of the present invention is to provide the technology for implementing video or digital cameras whose depth dimension is reduced while high image quality is ensured.
The gravest bottleneck in diminishing the depth dimension of cameras is the thickness of an optical system, especially a zoom lens system from the surface located nearest to its object side to an image pickup plane. To make use of a collapsible lens mount that allows the optical system to be taken out of a camera body for photo-taking and received therein for carrying now becomes mainstream. However, the thickness of an optical system received in a collapsible lens mount varies largely with the lens type or filters used. Especially in the case of a so-called +precedent type zoom lens wherein a lens group having positive refracting power is positioned nearest to its object side, the thickness of each lens element and dead space are too large to set such requirements as zoom ratios and F-numbers at high values; in other words, the optical system does not become thin as expected, even upon received in the lens mount (JP-A 11-258507). A xe2x88x92 precedent type zoom lens, especially of two or three-group construction is advantageous in this regard. However, this type zoom lens, too, does not become slim upon received in a collapsible lens mount, even when the lens positioned nearest to the object side is formed of a positive lens (JP-A 11-52246), because the lens groups are each composed of an increased number of lens elements, and the thickness of lens elements is large. Among zoom lenses known so far in the art, those set forth typically in JP-A""s 11-287953, 2000-267009 and 2000-275520 are suitable for use with electronic imaging systems with improved image-formation capabilities including zoom ratios, angles of view and F-numbers, and may possibly be reduced in thickness upon received in collapsible lens mounts.
To make the first lens group thin, it is preferable to make an entrance pupil position shallow; however, the magnification of the second lens group must be increased to this end. For this reason, some considerable load is applied on the second lens group. Thus, it is not only difficult to make the second lens group itself thin but it is also difficult to make correction for aberrations. In addition, the influence of production errors grows. Thickness and size reductions may be achieved by making the size of an image pickup device small. To ensure the same number of pixels, however, the pixel pitch must be diminished and insufficient sensitivity must be covered by the optical system. The same goes true for the influence of diffraction.
To obtain a camera body whose depth dimension is reduced, a rear focusing mode wherein the rear lens group is moved for focusing is effective in view of the layout of a driving system. It is then required to single out an optical system less susceptible to aberration fluctuations upon rear focusing.
In view of such problems as referred to above, the primary object of the invention is to thoroughly slim down a video or digital camera by singling out a zoom mode or zoom construction wherein a reduced number of lens elements are used to reduce the size of a zoom lens and simplify the layout thereof and stable yet high image-formation capabilities are kept over an infinite-to-nearby range, and optionally making lens elements thin thereby reducing the total thickness of each lens group and slimming down a zoom lens thoroughly by selection of filters.
According to the present invention, the aforesaid object is achievable by the provision of a zoom lens comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power, wherein for zooming from a wide-angle end to a telephoto end of the zoom lens upon focused on an infinite object point, the second lens group moves toward the object side alone and the third lens group moves in a locus different from that of the second lens group with a varying spacing between adjacent lens groups, wherein:
the second lens group comprises two lens components, i.e., an object side-lens component and an image side-lens component, one of which is composed of a cemented lens component consisting of a positive lens element and a negative lens element and the other consists only of a positive single lens component, and
the object side-lens component satisfies condition (1):
0.6 less than R2FR/R2FF less than 1.05xe2x80x83xe2x80x83(1) 
where R2FF is the axial radius of curvature of the object side-surface of the object side-lens component in the second lens group, and R2FR is the axial radius of curvature of the image side-surface of the object side-lens component in the second lens group.
The advantages of, and the requirements for, the aforesaid zoom lens arrangement are now explained.
The zoom lens of the present invention comprises, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power. For zooming from the wide-angle end to the telephoto end of the zoom lens upon focused on an infinite object point, the second lens group moves toward the object side alone and the third lens group moves in a locus different from that of the second lens group with a varying spacing between the adjacent lens groups. The second lens group comprises two lens components, i.e., an object side-lens component and an image side-lens component, one of which is composed of a cemented lens component consisting of a positive lens element and a negative lens element and the other consists only of a positive single lens component (a two-component, three-lens arrangement).
Of two such lens components, the object side-lens component should preferably satisfy condition (1).
0.6 less than R2FR/R2FF less than 1.05xe2x80x83xe2x80x83(1) 
Here R2FF is the axial radius of curvature of the object side-surface of the object side-lens component in the second lens group, and R2FR is the axial radius of curvature of the image side-surface of the object side-lens component in the second lens group.
In the present disclosure, the term xe2x80x9ccemented lensxe2x80x9d should be understood to comprise a plurality of lens elements wherein a lens element formed of a single medium is thought of as one unit, and the xe2x80x9clens componentxe2x80x9d should be understood to refer to a lens group with no air separation therein, i.e., a single lens or a cemented lens.
For reductions in the size of a two-group zoom lens of xe2x88x92+ construction commonly used as the zoom lens for long-standing silver-halide film cameras, it is preferable to increase the magnification of the positive rear group (the second lens group) at each focal length. To this end, it is already well known to locate an additional positive lens component as the third lens group on the image side of the second lens group, wherein the spacing between the second lens group and the third lens group is varied for zooming from the wide-angle end to the telephoto end. The third lens group has also the possibility of being used for focusing.
To attain the object of the invention, i.e., to diminish the total thickness of a lens portion upon received in a collapsible mount yet perform focusing at the third lens group, it is an essential requirement to reduce fluctuations of off-axis aberrations inclusive of astigmatism. To this end, the second lens group should be composed of two lens components, i.e., consist of, in order from its object side, an object side-lens component and an image side-lens component, one of which is composed of a cemented lens component consisting of a positive lens element and a negative lens element and the other consists only of a positive single lens element.
For focusing at the third lens group, aberration fluctuations become a problem. However, the incorporation of an aspheric surface in the third lens group in an amount than required is not preferable. This is because, to take advantage of that aspheric surface, astigmatism remaining at the first and second lens groups must be corrected at the third lens group. If, in this state, the third lens group moves for focusing, then aberrations are out of balance. Accordingly, when focusing is performed at the third lens group, astigmatism must be eradiated at the first and second lens group all over the zoom range.
It is thus preferable that the third lens group is constructed of a spherical lens component or a reduced amount of asphericity, an aperture stop is located on the object side of the second lens group, and the second lens group is composed of two lens components, that is, an object side-lens component and an image side-lens component, one of which is composed of a cemented lens component consisting of a positive lens element and a negative lens element and the other consists only of a positive single lens component (three lens elements in all).
Exceeding the upper limit to condition (1) may be favorable for correction of spherical aberrations, coma and astigmatism throughout the zoom lens, but causes the effect of cementing on slacking the sensitivity to decentration to become slender. As the lower limit is not reached, correction of spherical aberrations, coma and astigmatism throughout the zoom lens tends to become difficult.
More preferably,
0.65 less than R2FR/R2FF less than 1.0xe2x80x83xe2x80x83(1)xe2x80x2
Most preferably,
0.7 less than R2FR/R2FF less than 0.95xe2x80x83xe2x80x83(1)xe2x80x3
Of the aforesaid two lens components, it is preferable that the cemented lens component is defined by the object side-lens component with satisfaction of conditions (2), (3) and (4).
0 less than L/R2FC less than 0.8xe2x80x83xe2x80x83(2) 
0.01 less than n2FNxe2x88x92n2FP less than 0.2xe2x80x83xe2x80x83(3) 
v2FN less than 26.5xe2x80x83xe2x80x83(4) 
Here L is the diagonal length of a (substantially rectangular) effective image pickup area of an image pickup device, R2FC is the axial radius of curvature of a cementing surface in the object side-lens component in the second lens group, n2FP and n2FN are the d-line refractive indices of the positive and negative lens elements of the object side-lens component in the second lens group, respectively, and V2FN is the d-line based Abbe number of the negative lens element of the object side-lens component in the second lens group.
Condition (2) provides a definition of correction of longitudinal chromatic aberration and chromatic aberration of magnification. Exceeding the upper limit of 0.8 to this condition may make it easy to increase the thickness of the cemented lens component in the second lens group, but renders correction of the longitudinal chromatic aberration difficult. Falling short of the lower limit of 0 may be favorable for correction of the longitudinal chromatic aberration, but offers an obstacle to diminishing the thickness of the zoom lens upon received in a collapsible lens mount because there is no option but to make the cemented lens component thick.
Condition (3) defines a difference in the index of refraction of the medium between the positive and the negative lens element of the object side-lens component in the second lens group. Falling short of the lower limit of 0.01 may be effective for reducing the relative decentration sensitivity between the two lens components in the second lens group, but renders general correction of coma and so on difficult. Exceeding the upper limit of 0.2 may be favorable for correction of aberrations all over the zooming range, but is unfavorable for improvements in the relative decentration sensitivity between the two lens components in the second lens group.
Condition (4) provides a definition of correction of longitudinal chromatic aberration and chromatic aberration of magnification. Exceeding the upper limit of 26.5 incurs under-correction of the longitudinal chromatic aberration. Although there is no particular lower limit to V2FN because of the absence of any practically suitable medium, a prima facie lower limit thereto may be 20. A glass material less than the lower limit of 20 costs much.
More preferably, at least one or all of the following conditions (2)xe2x80x2, (3)xe2x80x2 and (4)xe2x80x2 should be satisfied.
0.05 less than L/R2FC less than 0.75xe2x80x83xe2x80x83(2)xe2x80x2
0.02 less than n2FNxe2x88x92n2FP less than 0.18xe2x80x83xe2x80x83(3)xe2x80x2
v2FN less than 26xe2x80x83xe2x80x83(4)xe2x80x2
Even more preferably, at least one of the following conditions (2)xe2x80x3, (3)xe2x80x3 and (4)xe2x80x3 should be satisfied.
0.1 less than L/R2FC less than 0.7xe2x80x83xe2x80x83(2)xe2x80x3
0.03 less than n2FNxe2x88x92n2FP less than 0.16xe2x80x83xe2x80x83(3)xe2x80x3
v2FN less than 25.5xe2x80x83xe2x80x83(4)xe2x80x3
Most preferably, all conditions (2)xe2x80x3, (3)xe2x80x3 and (4)xe2x80x3 should be satisfied.
When the object side-lens component is defined by a cemented lens component consisting of a positive lens element and a negative lens component and the image side-lens component is composed of a positive single lens component, it is preferable to satisfy condition (5) with respect to the positive lens element located nearest to the image side of the second lens group.
xe2x88x921.0 less than (R2RF+R2RR)/(R2RFxe2x88x92R2RR) less than 0.6xe2x80x83xe2x80x83(5) 
Here R2RF and R2RR are the axial radii of curvature of the object side-surface and the image side-surface of the image side-lens component in the second lens group, respectively.
An aspheric surface is introduced to the air contact surface side of the (cemented) positive lens element in the second lens group, thereby decreasing the F-number. Nonetheless, as the lower limit of xe2x88x921.0 to condition (5) is not reached, spherical aberrations are likely to occur. As the upper limit of 0.6 is exceeded, astigmatism cannot fully be corrected even with the introduction of the aspheric surface into the first lens group.
More preferably,
xe2x88x920.7 less than (R2RF+R2RR)/(R2RFxe2x88x92R2RR) less than 0.34xe2x80x83xe2x80x83(5)xe2x80x2
Even more preferably,
0.025 less than (R2RF+R2RR)/(R2RFxe2x88x92R2RR) less than 0.34xe2x80x83xe2x80x83(5)xe2x80x3
A total of two aspheric lenses should preferably be used for correction of aberrations throughout the zoom lens; one in the first lens group (for the purpose of correction of distortion, astigmatism and coma) and one in the second lens group (for the purpose of correction of spherical aberrations). The use of three or more aspheric lenses is less effective, only to cost much.
To reduce fluctuations of off-axis aberrations upon zooming from the wide-angle end to the telephoto end, the third lens group should preferably move in a convex locus toward the image side of the zoom lens.
In consideration of correction of spherical aberrations, condition (6) should preferably be satisfied in addition to condition (5).
5 less than (R2FF+R2FR)/(R2FFxe2x88x92xe2x88x92R2FR) less than 60xe2x80x83xe2x80x83(6) 
Here R2FF is the axial radius of curvature of the surface located nearest to the object side of the object side-lens component in the second lens group, and R2FR is the axial radius of curvature of the surface located nearest to the image side of the object side-lens component in the second lens group.
As the upper limit of 60 to condition (6) is exceeded, spherical aberrations tend to remain under-corrected and lens thickness tends to increase. In addition, the processability of the positive lens in the object side-lens component becomes worse. As the lower limit of 5 is not reached, higher-order spherical aberrations rather occur and the processability of a deep concave surface on the negative lens side becomes worse.
More preferably,
7 less than (R2FF+R2FR)/(R2FFxe2x88x92R2FR) less than 60xe2x80x83xe2x80x83(6)xe2x80x2
Even more preferably,
8 less than (R2FF+R2FR)/(R2FFxe2x88x92R2FR) less than 60xe2x80x83xe2x80x83(6)xe2x80x3
Satisfaction of conditions (7) and (8) in addition to condition (5) is favorable for an exit pupil position, i.e., shading.
0.1 less than f2R/f3O less than 1.2xe2x80x83xe2x80x83(7) 
0.01 less than d2FRxc3x97R2FR/t22 less than 0.6xe2x80x83xe2x80x83(8) 
Here f2R is the focal length of the image side-lens component in the second lens group, f30 is the focal length of the third lens group, d2FR is the spacing between the image side-surface of the object side-lens component and the object side-surface of the image side-lens component in the second lens group, R2FR is the axial radius of curvature of the image side-surface of the object side-lens component in the second lens group, and t2 is the axial distance of the second lens group from the surface located nearest to the object side thereof to the surface located nearest to the image side thereof.
Exceeding the upper limit of 1.2 to condition (7) may be favorable for the exit pupil position at the wide-angle end, i.e., shading, but causes the amount of fluctuations of the exit pupil position with zooming to the telephoto end to become unacceptably large for shading at the telephoto end. As the lower limit of 0.1 is not reached, the exit pupil comes too close to the wide-angle end, often resulting in shading, and the amount of movement of the third lens group for focusing becomes too large, leading to the need of an excessive space. In addition, the principal point position of the second lens group moves back due to the need of giving strength to the positive lens located on the image side of the second lens group where axial paraxial rays are at high positions. Consequently, any high magnification is hardly obtainable and the first lens group tends to become huge.
Falling short of the lower limit of 0.01 to condition (8) is not only unfavorable for correction of astigmatism but is also prone to incur shading by reason of the exit pupil position at the wide-angle end. Exceeding the upper limit of 0.6 offers an obstacle to reducing the thickness of the zoom lens upon received in a collapsible lens mount because the thickness of the second lens group increases.
More preferably,
0.15 less than f2R/f3O less than 1.0xe2x80x83xe2x80x83(7)xe2x80x2
0.03 less than d2FRxc3x97R2FR/t22 less than 0.3xe2x80x83xe2x80x83(8)xe2x80x2
Even more preferably,
0.3 less than f2R/f3O less than 0.8xe2x80x83xe2x80x83(7)xe2x80x3
0.05 less than d2FRxc3x97R2FR/t22 less than 0.21xe2x80x83xe2x80x83(8)xe2x80x3
Apart from these conditions, satisfaction of condition (9) in addition to condition (5) or (6) should be preferable for reducing the size of the zoom lens upon received in a collapsible lens mount.
0.2 less than R2FC/f2F less than 2xe2x80x83xe2x80x83(9) 
Here R2FC is the radius of curvature of the cementing surface in the object side-lens component in the second lens group, and f2F is the focal length of the object side-lens component in the second lens group.
As the lower limit of 0.2 to condition (9) is not reached, it is easy to make the object side-lens component in the second lens group thin; however, it is difficult to make correction of longitudinal chromatic aberration. Exceeding the upper limit of 2 may be favorable for correction of the longitudinal chromatic aberration, but offers an obstacle to reducing the thickness of the zoom lens upon received in a collapsible lens mount because there is no option but to increase the thickness of the object side-lens component.
More preferably,
0.3 less than R2FC/f2F less than 1.6xe2x80x83xe2x80x83(9)xe2x80x2
Even more preferably,
0.4 less than R2FC/f2F less than 1.2xe2x80x83xe2x80x83(9)xe2x80x3
Preferably for size reductions of the zoom lens upon received in a collapsible lens mount, any one or two or more of conditions (a), (b) and (c) should be satisfied in addition to condition (5) or (6) or (9).
0.0 less than f2/f2R less than 1.3xe2x80x83xe2x80x83(a) 
0.04 less than t2N/t2 less than 0.2xe2x80x83xe2x80x83(b) 
0.5 less than t2/L less than 1.2xe2x80x83xe2x80x83(c) 
Here f2 is the composite focal length of the second lens group, f2R is the focal length of the image side-lens component in the second lens group, t2N is the axial distance of the second lens group from the image side-surface of the cemented positive lens element of the object side-lens component to the image side-surface of the negative lens element thereof, t2 is the axial distance of the second lens group from the surface located nearest to the object side thereof to the surface located nearest to the image side thereof, and L is the diagonal length of a (substantially rectangular) image pickup area of an image pickup device.
Condition (a) defines the ratio between the focal length of the positive lens element on the image side of the second lens group and the composite focal length of the second lens group. As the upper limit of 1.3 to that condition is exceeded, the magnification of the second lens group cannot be made high because the principal point of the second lens group comes close to the image side of the zoom lens. This in turn may cause the amount of movement, and the size, of the first lens group to become large, or bring about a dead space in the rear of the second lens group when the zoom lens is used, resulting in an increase in the length of the zoom lens or rendering complicated, huge or thick the mechanical structure of the lens barrel for reducing the thickness of the zoom lens upon received in a collapsible lens mount. As the lower limit of 0.0 is not reached, correction of astigmatism becomes difficult.
Condition (b) defines the axial distance t2N of the second lens group from the image side-surface of the cemented positive lens element of the object side-lens component in the second lens group to the image side-surface of the negative lens element thereof. Unless this part has a certain thickness, full correction of astigmatism is unachievable; however, it offers an obstacle to reducing the thickness of each element in the optical system. For this reason, the astigmatism is corrected by the introduction of an aspheric surface to any surface in the first lens group. Nonetheless, as the lower limit of 0.04 is not reached, the astigmatism cannot fully be corrected. As the upper limit of 0.2 is exceeded, thickness increases unacceptably.
More preferably, any one of the following conditions (a)xe2x80x2, (b)xe2x80x2 and (c)xe2x80x2 should be satisfied, or two or three thereof should simultaneously be satisfied.
0.5 less than f2/f2R less than 1.2xe2x80x83xe2x80x83(a)xe2x80x2
0.06 less than t2N/t2 less than 0.18xe2x80x83xe2x80x83(b)xe2x80x2
0.55 less than t2/L less than 1.1xe2x80x83xe2x80x83(c)xe2x80x2
Even more preferably, any one of the following conditions (a)xe2x80x3, (b)xe2x80x3 and (c)xe2x80x3 should be satisfied, or two or three thereof should simultaneously be satisfied.
0.9 less than f2/f2R less than 1.1xe2x80x83xe2x80x83(a)xe2x80x3
0.08 less than t2N/t2 less than 0.16xe2x80x83xe2x80x83(b)xe2x80x3
0.6 less than t2/L less than 1.0xe2x80x83xe2x80x83(c)xe2x80x3
At a zoom ratio of 2.3 or greater, satisfaction of conditions (d) and (e) makes some contribution to size reductions.
1.2 less than xe2x88x92xcex22t less than 2.0xe2x80x83xe2x80x83(d) 
1.6 less than f2/fW less than 3.0xe2x80x83xe2x80x83(e) 
Here xcex22t is the magnification of the second lens group at the telephoto end (upon focused at an infinite object point), f2 is the focal length of the second lens group, and fW is the focal length of the zoom lens at the wide-end angle (upon focused on an infinite object point).
Condition (d) defines the magnification xcex22t of the second lens group at the telephoto end (when the zoom lens is focused on an infinite object point). The absolute value of that magnification should preferably be as small as possible because an entrance pupil position at the wide-angle end is so shallow that the diameter and, hence, the thickness of the first lens group can be reduced. As the lower limit of 1.2 is not reached, it is difficult to satisfy thickness, and as the upper limit of 2.0 is exceeded, it is difficult to make correction for aberrations (spherical aberrations, coma and astigmatism).
Condition (e) defines the focal length f2 of the second lens group. That focal length should preferably be short to slim down the second lens group itself; however, too short a focal length is not preferable for correction of aberrations, because there is going to an unreasonable power profile where, for instance, the rear principal point of the first lens group is positioned on the image side of the zoom lens. As the lower limit of 1.6 is not reached, it is difficult to make correction for spherical aberrations, coma, astigmatism and so on, and as the upper limit of 3.0 is exceeded, size reductions are little achievable.
More preferably, the following conditions (d)xe2x80x2 and/or (e)xe2x80x3 should be satisfied.
1.25 less than xe2x88x92xcex22t less than 1.9xe2x80x83xe2x80x83(d)xe2x80x2
1.8 less than f2/fW less than 2.7xe2x80x83xe2x80x83(e)xe2x80x2
Even more preferably, the following conditions (d)xe2x80x3 and/or (e)xe2x80x3 should be satisfied.
1.3 less than xe2x88x92xcex22t less than 1.8xe2x80x83xe2x80x83(d)xe2x80x3
2.0 less than f2/fW less than 2.5xe2x80x83xe2x80x83(e)xe2x80x3
As already noted, size reductions are contradictory to correction of aberrations. It is thus preferable to introduce an aspheric surface to the positive lens located nearest to the object side of the second lens group. This is effective for correction of spherical aberrations and coma and, accordingly, correction of astigmatism and longitudinal chromatic aberration can favorably be made.
As already described, it is preferable that when rear focusing is carried out at the third lens group, correction of off-axis aberrations throughout the zooming range is substantially finished at the first lens group and the second lens group. This is achievable by determination of what type of lens arrangement is selected for the first lens group in view of the second lens group, as described below. The first type lens arrangement for the first lens group is composed of, in order from its object side, a negative lens subgroup comprising up to two negative lenses and a positive lens subgroup consisting of one single lens having positive refracting power, wherein at least one negative lens in the negative lens subgroup includes an aspheric surface and conditions (f) and (g), given below, are satisfied.
xe2x88x920.03 less than fW/R11 less than 0.4xe2x80x83xe2x80x83(f) 
0.15 less than dNP/fW/1.0xe2x80x83xe2x80x83(g) 
Here R11 is the axial radius of curvature of the first lens surface as counted from the object side of the first lens group, dNP is an axial air separation between the negative lens subgroup and the positive lens subgroup, and fW is the focal length of the zoom lens at the wide-angle end (upon focused on an infinite object point).
Condition (f) defines the radius of curvature of the first lens surface in the aforesaid first lens arrangement for the first lens group. Preferably, distortion is corrected by the aspheric surface introduced into the first lens group, and astigmatism is corrected by the remaining spherical component. Exceeding the upper limit of 0.4 is unfavorable for correction of astigmatism, and as the lower limit of xe2x88x920.03 is not reached, full correction of distortion is unachievable even at the aspheric surface.
Condition (g) defines the axial air separation dNP between the negative lens subgroup and the positive lens subgroup in the first type lens arrangement for the first lens group. Exceeding the upper limit of 1.0 may be favorable for correction of astigmatism, but causes the thickness of the first lens group to increase, contradictory to size reductions. As the lower limit of 0.15 is not reached, correction of astigmatism becomes difficult.
More preferably, the following conditions (f)xe2x80x2 and/or (g)xe2x80x2 be satisfied.
xe2x88x920.02 less than fW/R11 less than 0.24xe2x80x83xe2x80x83(f)xe2x80x2
0.18 less than dNP/fW/0.7xe2x80x83xe2x80x83(g)xe2x80x2
Even more preferably, the following conditions (f)xe2x80x3 and/or (g)xe2x80x3 be satisfied.
xe2x88x920.01 less than fW/R11 less than 0.16xe2x80x83xe2x80x83(f)xe2x80x3
0.2 less than dNP/fW/0.5xe2x80x83xe2x80x83(g)xe2x80x3
On the other hand, when the first lens group is composed of, in order from its object side, two negative meniscus lenses each convex on its object side and one positive lens, the introduction of an aspheric surface to any surface facing the air separation (the quantity of which is dNN as measured along the optical axis) between the two negative meniscus lenses is favorable for correction of distortion, astigmatism and coma. In view of the principal point position, it is further preferable satisfy conditions (h) and (i).
0.4 less than R12/R13 less than 1.3xe2x80x83xe2x80x83(h) 
0.02 less than dNN/fW less than 0.25xe2x80x83xe2x80x83(i) 
Here R12 is the axial radius of curvature of the image side-surface of the first negative meniscus lens as counted from the object side of the zoom lens and R13 is the axial radius of curvature of the object side-surface of the second negative meniscus lens; condition (h) defines the R12-to-R13 ratio. As the lower limit of 0.4 is not reached, there is no option but to increase dNN because distortion is prone to become worse and for the reason of lens interferences. Exceeding the upper limit of 1.3 is not only unfavorable for correction of astigmatism but also renders it difficult to configure the second negative meniscus lens.
The value of condition (i) should preferably be reduced as much as possible so long as lens interferences are permissible. As the upper limit of 0.25 is exceeded, however, correction of astigmatism becomes difficult because of the need of unreasonably diminishing dNP.
More preferably, the following conditions (h)xe2x80x2 and/or (i)xe2x80x2 should be satisfied.
0.47 less than R12/R13 less than 1.0xe2x80x83xe2x80x83(h)xe2x80x2
0.02 less than dNN/fW less than 0.2xe2x80x83xe2x80x83(i)xe2x80x2
Even more preferably, the following conditions (h)xe2x80x3 and/or (i)xe2x80x3 should be satisfied.
0.5 less than R12/R13 less than 0.8xe2x80x83xe2x80x83(h)xe2x80x3
0.02 less than dNN/fW less than 0.17xe2x80x83xe2x80x83(i)xe2x80x3
Alternatively, when the first lens group is composed of, in order from its object side, one negative meniscus lens convex on its object side and one positive lens, it is preferable to satisfy conditions (j) and (k) with respect to the first lens group.
xe2x88x925.0 less than (R1P1+R1P2)/(R1P1xe2x88x92R1P2) less than xe2x88x921.3xe2x80x83xe2x80x83(j) 
1.7 less than nd1N less than 1.95xe2x80x83xe2x80x83(k) 
Here R1P1 and R1P2 are the axial radii of curvature of the object and image sides of the positive lens component in the first lens group, respectively, and nd1N is the d-line refractive index of the negative meniscus lens in the first lens group.
Condition (j) defines the shape factor of the positive lens in the first lens group. Being less than the lower limit of xe2x88x925.0 is not only unfavorable for correction of astigmatism but also requires an extra spacing between the first lens group and the second lens group to avoid mechanical interferences during zooming. As the upper limit of xe2x88x921.3 is exceeded, correction of distortion tends to suffer inconvenience.
Condition (k) defines the refractive index of the medium of the negative lens in the first lens group. To ensure the strong negative power of the first lens group with one lens alone, R11 must have a negative, strong curvature. Even when an aspheric surface is introduced to this lens, correction of distortion becomes insufficient. Consequently, the refractive index of the medium should preferably be made as high as possible. As the lower limit of 1.7 is not reached, distortion tends to occur. The prima facie upper limit of 1.95 is set because of the absence of any practical glass material exceeding that, inclusive of chromatic aberrations (Abbe number).
More preferably, the following conditions (j)xe2x80x2 and/or (k)xe2x80x2 should be satisfied.
xe2x88x925.0 less than (R1P1+R1P2)/(R1P1xe2x88x92R1P2) less than xe2x88x921.7xe2x80x83xe2x80x83(j)xe2x80x2
1.72 less than nd1N less than 1.95xe2x80x83xe2x80x83(k)xe2x80x2
Even more preferably, the following conditions (j)xe2x80x3 and/or (k)xe2x80x3 should be satisfied.
xe2x88x925.0 less than (R1P1+R1P2)/(R1P1xe2x88x92R1P2) less than xe2x88x922.0xe2x80x83xe2x80x83(j)xe2x80x3
1.74 less than nd1N less than 1.95xe2x80x83xe2x80x83(k)xe2x80x3
The second type lens arrangement for the first lens group is composed of, in order from its object side, a single lens including one aspheric surface and having weak refracting power, one negative single lens and one positive single lens, and satisfies condition (1).
xe2x88x920.2 less than fW/f1* less than 0.3xe2x80x83xe2x80x83(1) 
Here f1* is the focal length of the lens in the first lens group, which includes an aspheric surface and has weak refracting power, and fW is the focal length of the zoom lens at the wide-angle end (upon focused on an infinite object point).
Condition (1) defines the focal length f1* of the lens including an aspheric surface and having weak refracting power in the second type lens arrangement for the first lens group. As the upper limit of 0.3 is exceeded, the power of the negative lens in the first lens group becomes too strong and distortion tends to become worse, and the negative lens is difficult to process because of too small a radius of curvature of its concave surface. Falling short of the lower limit of xe2x88x920.2 is not preferable for correction of astigmatism because the aspheric surface then works primarily for correction of distortion.
More preferably,
xe2x88x920.15 less than fW/f1* less than 0.2xe2x80x83xe2x80x83(1)xe2x80x2
Even more preferably,
xe2x88x920.1 less than fW/f1* less than 0.1xe2x80x83xe2x80x83(1)xe2x80x3
The third lens group should be composed of one positive single lens both surfaces of which are formed of substantially spherical surfaces, preferably with satisfaction of condition (m).
xe2x88x921 less than (R31+R32)/(R31xe2x88x92R32) less than 1xe2x80x83xe2x80x83(m) 
Here R31 and R32 are the radii of curvature of the object and image sides of the positive lens that forms the third lens group, respectively. As the upper limit of 1 to condition (m) is exceeded, fluctuations of astigmatism with rear focusing become too large and so astigmatism is likely to become worse with respect to a nearby object point, although astigmatism at an infinite object point may be well corrected. As the lower limit of xe2x88x921 is not reached, the fluctuations of astigmatism with rear focusing are reduced, but correction of aberrations with respect to an infinite object point becomes difficult.
More preferably,
xe2x88x920.45 less than (R31+R32)/(R31xe2x88x92R32) less than 0.5xe2x80x83xe2x80x83(m)xe2x80x2
Even more preferably,
xe2x88x920.25 less than (R31+R32)/(R31xe2x88x92R32) less than 0.5xe2x80x83xe2x80x83(m)xe2x80x3
To make the first and the second lens group thin while optimizing aberrations and paraxial quantity, the thickness of one lens group should preferably be reconciled with that of another lens group, as defined by conditions (n) and (o).
0.5 less than t2/t1 less than 1.5xe2x80x83xe2x80x83(n) 
0.4 less than t1/L less than 1.3xe2x80x83xe2x80x83(o) 
Here t1 is the axial distance of the first lens group from the lens surface located nearest to the object side thereof to the lens surface located nearest to the image side thereof, t2 is the axial distance of the second lens group from the lens surface located nearest to the object side thereof to the lens surface located nearest to the image side thereof, and L is the diagonal length of the (substantially rectangular) effective image pickup area of the image pickup device.
Condition (n) defines the thickness ratio between the first and the second lens group. Increasing any surface-to-surface spacing in each lens group may be effective for correction of off-axis aberrations, especially astigmatism; however, this is not acceptable for thickness reductions. It is the second lens group that is less susceptible to deterioration in off-axis aberrations with the effect of the aspheric surface, even when each surface-to-surface spacing in the lens group is reduced. In other words, the smaller the value of condition (n), the better the balance becomes. As the upper limit of 1.5 is exceeded, correction of off-axis aberrations such as astigmatism becomes insufficient with a decreasing thickness of each lens group. As the lower limit of 0.5 is not reached, the second lens group cannot physically be set up or, rather, the first lens group becomes thick.
Condition (o) defines the total thickness of the first lens group. Exceeding the upper limit of 1.3 offers an obstacle to thickness reductions, and as the lower limit of 0.4 is not reached, the radius of curvature of each lens surface must be slacked and so it is difficult to put paraxial relations in order and make correction for various aberrations.
More preferably,
0.6 less than t2/t1 less than 1.4xe2x80x83xe2x80x83(n)xe2x80x2
Even more preferably,
0.7 less than t2/t1 less than 1.3xe2x80x83xe2x80x83(n)xe2x80x3
To ensure an edge thickness and a mechanical space, it necessary to vary the more proper range for condition (o) with the value of L, as given by condition (o)xe2x80x2.
0.6 less than t1/L less than 1.3 provided that Lxc2x7fW less than 6.2 
0.5 less than t1/L less than 1.2 provided that 6.2 less than Lxc2x7fW less than 9.2 
0.4 less than t1/L less than 1.1 provided that 9.2 less than Lxc2x7fW 
Desirously, the zoom lens of the present invention, wherein off-axis chief rays can be directed substantially vertically to the image plane, is used with an electronic imaging system comprising an electronic image pickup device located on the image plane side of the zoom lens.
To make a sensible tradeoff between good image quality and size reductions, it is desired that the diagonal length L of the effective image pickup area of the image pickup device be in the range of 3.0 mm to 12.0 mm inclusive. As the lower limit of 3.0 mm is not reached or the size of the image pickup device becomes too small, it is difficult to cover a shortfall in sensitivity. As the upper limit of 12.0 mm is exceeded or the size of the image pickup device becomes too large, the size of the zoom lens tends to become large incidentally; the effect of the present invention on slimming-down becomes slender. While, in Examples 1 through 7 given later, the focal length at the wide-angle end is standardized at 1, it is preferable that the diagonal length of the effective image pickup area of the image pickup device is properly determined as shown in Example 8.
The zoom lens of the present invention is favorable for setting up an electronic imaging system including a wide-angle area. In particular, the present zoom lens is preferable for use on an electronic imaging system wherein the diagonal half angle of view, xcfx89w, at the wide-angle end satisfies the following condition (this diagonal half angle of view is tantamount to the wide-angle-end half angle of view xcfx89W referred to in the examples given later):
27xc2x0 less than xcfx89W less than 42xc2x0
Being less than the lower limit of 27xc2x0 to this condition or the wide-angle-end half angle of view becoming narrow is advantageous for correction of aberrations; however, this wide-angle-end half angle is no longer practical. As the upper limit of 42xc2x0 is exceeded, on the other hand, distortion and chromatic aberration of magnification tend to occur and the number of lens elements increases.
Thus, the present invention provides means for improving the image-formation capability of the zoom lens part while diminishing the thickness the zoom lens part upon received in a collapsible lens mount.
Next, how and why the thickness of filters is reduced is now explained. In an electronic image pickup system, an infrared absorption filter having a certain thickness is usually inserted between an image pickup device and the object side of a zoom lens, so that the incidence of infrared light on the image pickup plane is prevented. Here consider the case where this filter is replaced by a coating devoid of thickness. In addition to the fact that the system becomes thin as a matter of course, there are spillover effects. When a near-infrared sharp cut coat having a transmittance of at least 80% at 600 nm and a transmittance of up to 10% at 700 nm is introduced between the image pickup device in the rear of the zoom lens system and the object side of the system, the transmittance on the red side is relatively higher as compared with those of the absorption type, so that the tendency of bluish purple to turn into magentaxe2x80x94a defect of a CCD or other solid-state image pickup device having a complementary colors mosaic filterxe2x80x94is diminished by gain control and there can be obtained color reproduction comparable to that by a CCD or other solid-state image pickup device having a primary colors filter. On the other hand, a complementary colors filter is higher in substantial sensitivity and more favorable in resolution than a primary colors filter-inserted CCD due to its high transmitted light energy, and provides a great merit when used in combination with a small-size CCD. Regarding an optical low-pass filter that is another filter, too, its total thickness tLPF (mm) should preferably satisfy condition (p):
0.15 less than tLPF/a less than 0.45 (mm)xe2x80x83xe2x80x83(p) 
Here a is the horizontal pixel pitch (in xcexcm) of the image pickup device.
Reducing the thickness of the optical low-pass filter, too, is effective for making the thickness of the zoom lens upon received in a collapsible mount; however, this is generally not preferred because the moirxc3xa9 preventive effect becomes slender. On the other hand, as the pixel pitch becomes small, the contrast of frequency components greater than Nyquist threshold decreases under the influence of diffraction of an image-formation lens system and, consequently, the decrease in the moirxc3xa9 preventive effect is more or less acceptable. For instance, it is known that when three different filters having crystallographic axes in directions where upon projected onto the image plane, the azimuth angle is horizontal (=0xc2x0) and xc2x145xc2x0 are used while they are put one upon another, some moirxc3xa9 preventive effect is obtainable. According to the specifications known to make the filter assembly thinnest, each filter is displaced by a xcexcm in the horizontal and by SORT(xc2xd)*a xcexcm in the xc2x145xc2x0 directions. Here SORT means a square root. The then filter thickness is approximately given by [1+2*SQRT(xc2xd)]*a/5.88 (mm).
This is the specification where the contrast is reduced down to zero at a frequency corresponding just to Nyquist threshold. At a thickness a few % to a few tens of % smaller than this, a little more contrast of the frequency corresponding to Nyquist threshold appears; however, this can be suppressed under the influence of the aforesaid diffraction. In other filter embodiments where two filters are placed one upon another or one single filter is used, too, it is preferable to meet condition (p). When the upper limit of 0.45 is exceeded, the optical low-pass filter becomes too thick, contrary to size reduction requirements. When the lower limit of 0.15 is not reached, moirxc3xa9 removal becomes insufficient. In this condition, a should be 5 xcexcm or less.
When a is 4 xcexcm or less or where the optical low-pass filter is more susceptible to diffraction, it is preferable that
0.13 less than tLPF/a less than 0.42 (mm)xe2x80x83xe2x80x83(p)xe2x80x2
When a is equal to or greater than 4 xcexcm, condition (p)xe2x80x2 may be rewritten as (p)xe2x80x3.
0.3 less than tLPF/a less than 0.4 (mm) provided that three filters are placed one upon another and a less than 5 xcexcm,
0.2 less than tLPF/a less than 0.16 (mm) provided that two filters are placed one upon another and a less than 5 xcexcm, and
0.1 less than tLPF/a less than 0.16 (mm) provided that one filter is used and a less than 5 xcexcm.
When a is equal to or less than 4 xcexcm, condition (p)xe2x80x2 may be rewritten as (p)xe2x80x3.
0.25 less than tLPF/a less than 0.37 (mm) provided that three filters are placed one upon another and a less than 4 xcexcm,
0.16 less than tLPF/a less than 0.25 (mm) provided that two filters are placed one upon another and a less than 4 xcexcm, and
0.08 less than tLPF/a less than 0.14 (mm) provided that one filter is used and a less than 4 xcexcm.
When an image pickup device having a small pixel pitch is used, there is degradation in image quality under the influence of diffraction effect by stop-down. In this case, the electronic image pickup system is designed in such a way as to have a plurality of apertures each of fixed aperture size, one of which can be inserted into any one of optical paths between the lens surface located nearest to the image side of the first lens group and the lens surface located nearest to the object side of the third lens group and can be replaced with another as well, so that illuminance on the image plane can be adjusted. Then, media whose transmittances with respect to 550 nm are different but less than 80% are filled in some of the plurality of apertures for light quantity control. Alternatively, when control is carried out in such a way as to provide a light quantity corresponding to such an F-number as given by a (xcexcm)/F-number less than 0.4, it is preferable to fill the apertures with medium whose transmittance with respect to 550 nm are different but less than 80%. In the range of the full-aperture value to values deviating from the aforesaid condition as an example, any medium is not used or dummy media having a transmittance of at least 91% with respect to 550 nm are used. In the range of the aforesaid condition, it is preferable to control the quantity of light with an ND filter or the like, rather than to decrease the diameter of the aperture stop to such an extent that the influence of diffraction appears.
Alternatively, it is acceptable to uniformly reduce the diameters of a plurality of apertures inversely with the F-numbers, so that optical low-pass filters having different frequency characteristics can be inserted in place of ND filters. As degradation by diffraction becomes worse with stop-down, it is desirable that the smaller the aperture diameter, the higher the frequency characteristics the optical low-pass filters have.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts that will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.